Process for producing leather-like thermoplastic polyester series resin sheet

ABSTRACT

A leather-like thermoplastic polyester series resin sheet is disclosed, including a uniformly and finely foamed sheet comprising a thermoplastic polyester series resin, wherein said foamed sheet has cells having a diameter of substantially not longer than 1 mm, the cell walls thereof have a thickness of substantially not thicker than 100 μm, and the cell wall constructing each cell extends with irregular bending so as to make a flexibility of not more than 20,000 mg in the Gurley flexibility test. A process for production of the leather-like thermoplastic polyester series resin sheet is also disclosed.

This application is continuation of application Ser. No. 773,005, filedOct. 8, 1991 and now abandoned, which in turn is a division ofapplication Ser. No. 699,072, filed May 13, 1991 and now U.S. Pat. No.5,110,844.

FIELD OF THE INVENTION

The present invention relates to a leather-like sheet made of athermoplastic polyester series resin (hereinafter, is referred to asPAT) and also a process for production of the same.

BACKGROUND OF THE INVENTION

Leather-like sheets made of a synthetic resin are already known. Aleather-like synthetic resin sheet is generally called "artificialleather". Conventional artificial leathers are mainly prepared bysticking a surface layer of a foamed synthetic resin onto a basic fabriclayer made of fibers and integrating them with each other.

The reason of requiring the basic fabric layer for preparing theconventional artificial leather is that the use of the foamed resinlayer only cannot impart the properties, in particular, the tensilestrength comparable to a leather to the artificial leather.

Due to the use of the basic fabric layer, a conventional artificialleather has a problem that the basic fabric layer peels off from theresin layer and also has a disadvantage that the production step becomescomplicated.

Hitherto, as the synthetic resin for making artificial leathers,polyvinyl chloride series resins and polyurethane series resins havebeen mainly used since these resins show proper flexibility and can beeasily foamed. Also, a polyamide resin was considered possible to use asa synthetic resin for an artificial leather but since it was difficultto foam the polyamide resin, the resin had not been widely used forartificial leathers. Other resins were originally considered to beunsuitable for making artificial leathers or it was difficult to foamthese resins and, hence, these resins have never been used for theproduction of artificial leathers.

On the other hand, PAT has been once formed into fibers, compounded andthen used for the production of an artificial leather in place of abasic fabric layer. However, PAT has never been used as a resin layerfor an artificial leather since it is difficult to foam PAT. Morespecifically, the reason is that since PAT is hard, poor in flexibilityand crystalline as well as has the characteristic that PAT suddenlychanges from a solid to a liquid having a low viscosity at heating, thetemperature range showing a viscosity suitable for foaming is narrow,thereby the temperature control at foaming is difficult and also theresin immediately releases gases which become the motive power forfoaming, which results in making it difficult to foam the resin.

However, it is known that when PAT containing a foaming agent isextruded in the air from an extruder, it is foamed although theexpansion ratio is low. The expansion ratio is usually no higher thantwice. When the resin is extruded in a sheet form, the resin can befoamed at a relatively large expansion ratio but even in such a case,the expansion ratio is about 4 times. Thus, it has been attempted tofoam PAT at a larger expansion ratio.

JP-A-55-2045 (the term "JP-A" as used herein means an "unexaminedpublished Japanese patent application") describes a process of foamingat a high expansion ratio by extruding PAT containing a foaming agent ina region having a reduced pressure. According to the process describedin this patent, it is reported that a foamed material of a circularcross section foamed at an expansion ratio of from 22 times to 32 timeswas obtained. However, the process has a fault that it is difficult toconstruct a reduced pressure zone. In particular, it is difficult toundergo this process when it is intended to make an artificial leather.The reason is as follows. That is, since when PAT is extruded in a sheetform, the extruded sheet has an uneven surface by foaming and sometimesis largely deformed by foaming, it is not easy to construct a reducedpressure zone capable of continuously passing therethrough the sheethaving such an irregular form and in particular, it is difficult toachieve sufficient reduced-pressure sealing for a sheet form material.

On the other hand, it is known that when a foamed plate made of astyrene series resin is bent, the foamed plate becomes rich inflexibility as described in JP-B-63-60705 (the term "JP-B" as describedherein means an "examined published Japanese patent application").However, the resin to be used in this patent is limited to a styreneseries resin and the patent publication teaches nothing about a foamedplate of PAT. Also, the patent publication teaches only that theflexible foamed plate thus obtained is suitable for a material beingused by bending as a cushioning material, a heat insulating material, ora packing material and teaches nothing about a material becoming aleather.

The present inventors have attempted to make an artificial leather byfoaming PAT which was generally considered reluctant to foam and triedto foam PAT by extruding PAT in the air. As the result, it has beenfound that when PAT is extruded under a proper condition, a sheet foamedat about 4 times is obtained. Furthermore, the inventors have discoveredthat when the foamed sheet thus obtained is heated by being brought intocontact with steam, etc., the sheet is further expanded to become asheet uniformly and finely foamed at a higher expansion ratio.

Also, the inventors have further discovered that when the uniformly andfinely foamed PAT sheet thus obtained is compressed by holding the sheetbetween rolls at normal temperature, the sheet increases theflexibility, whereby a leather-like flexible, high tensile strength,high tear strength and tough sheet is obtained. Furthermore, theinventors have confirmed that when the above-described uniformly andfinely foamed PAT sheet is bent or crumpled in place of compressing byholding the sheet between rolls, a leather-like sheet is similarlyobtained.

Also, as the result of investigating the structure of the leather-likesheet obtained as described above, it has been confirmed that theleather-like sheet is uniformly and finely foamed as well as the cellwalls are in a specific state and also the whole sheet has a specificflexibility.

In this case, the term "uniformly and finely foamed" means that thecells are small ones having a diameter of substantially 1 mm or shorterand the cell walls are in a state having a thickness of not thicker than100 μm.

Also, the term "the cell walls are in a specific state" means that atleast part of the cell walls extends while irregularly bending.Moreover, the term "the whole sheet has a specific flexibility" meansthat the sheet has a small value of flexibility by the Gurleyflexibility test of no higher than 20,000 mg. Furthermore, it has beenfound that when the leather-like sheet is immersed in a strong alkalisolution, the cells become continuous to render the sheet breathable andwater absorptive.

The present invention has been made based on the above-describedknowledges.

SUMMARY OF THE INVENTION

That is, the present invention has been made for obtaining a sheethaving a property comparable to a leather by the sheet only obtained byfoaming PAT.

An object of the present invention is to provide a sheet comprising afoamed sheet of PAT having the appearance and properties of naturalleather.

Other object of the present invention is to provide a sheet comprising afoamed sheet of PAT having the properties of a flexible, high tearstrength and tough leather.

Also, a still other object of the present invention is a sheet providedwith continuous cells, which has the properties similar to a naturalleather having breathability and water absorbing property.

A further object of the present invention is to provide a process foreasily producing a foamed sheet of PAT having the properties of aflexible, high tear strength and tough leather.

Another object of the present invention is to provide a process forproducing a foamed sheet of PAT having the properties of a leatherhaving breathability and water absorbing property by immersing thefoamed sheet in a strong alkali solution to form continuous cells.

As described above, the present invention includes an invention about aproduct and an invention of a process for production of the same. Theinvention of the product relates to a leather-like PAT sheet and theinvention of the process for production relates to a process forproduction of a leather-like PAT sheet.

Thus, according to an embodiment of the present invention, there isprovided a leather-like PAT sheet, wherein the sheet has fine cellshaving a diameter of substantially not larger than 1 mm, the cell wallshave a thickness of substantially not thicker than 100 μm, the cell wallof each cell extends while irregularly bending, and the flexibility as asheet by the Gurley flexibility test is not more than 20,000 mg.

Also, according to another embodiment of the present invention, there isprovided a process for production of a leather-like PAT sheet, whichcomprises applying the force of compression or bending to a uniformlyand finely foamed PAT sheet, in which the foamed sheet has cells havinga diameter of substantially not longer than 1 mm and the cell walls havea thickness of substantially not thicker than 100 μm, to make the cellwall constituting each cell a state of extending while irregularlybending.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged schematic cross section of the leather-like PATfoamed sheet of the present invention in the thickness direction.

FIG. 2 is a microscopic photograph (magnification: about 35 times)showing an enlarged cross section of one example of the leather-like PATfoamed sheet of the present invention in the thickness direction.

FIG. 3 is an enlarged schematic cross section of a conventional PATfoamed sheet in the thickness direction.

FIG. 4 is a microscopic photograph (magnification: about 50 times)showing an enlarged cross section of one example of a conventional PATfoamed sheet in the thickness direction.

FIG. 5 is a microscopic photograph (magnification: about 35 times)showing an enlarged cross section of one example of the leather-like PATfoamed sheet having been treated with an alkali solution.

FIG. 6 is a graph showing comparatively the ductility of theleather-like PAT foamed sheet of the present invention and the ductilityof a conventional PAT foamed sheet.

FIG. 7 to FIG. 9 are schematic views showing various embodiments ofcarrying out secondary foaming in the process of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

PAT which can be used in the present invention is a high molecularweight chain polyester obtained by reaction of an aromatic dicarboxylicacid and a dihydric alcohol.

As the aromatic dicarboxylic acid, terephthalic acid is most frequentlyused but isophthalic acid or 2,6-naphthalenedicarboxylic acid issometimes used. Furthermore, diphenyl ether dicarboxylic acid, diphenylsulfone dicarboxylic acid, or diphenoxydicarboxylic acid is, as the casemay be, used.

On the other hand, as the dihydric alcohol, ethylene glycol is mainlyused but trimethylene glycol, tetramethylene glycol, neopentyleneglycol, hexamethylene glycol, cyclohexanedimethylol,tricyclodecanedimethylol, 2,2-bis(4-β-hydroxyethoxyphenyl)propane,4,4'-bis(β-hydroxyethoxy)diphenyl sulfone, or diethylene glycol may beused.

The above-described PAT is commercially available and in the presentinvention, commercially available PAT can be used.

Examples of PAT which is suitably used in the present inventionpolyethylene terephthalate, polybutylene terephthalate, a polybutyleneterephthalate elastomer, non-crystalline polyesters, and polycyclohexaneterephthalate.

These resins can be used singly or as a mixture thereof. Also, theabove-described resin(s) can be used as a mixture thereof and otherresin than PAT. When other resin than PTA is used as a mixture with PAT,the content of the resin is less than that of PAT.

The leather-like sheet in the present invention must be a uniformly andfinely foamed sheet. This can be defined by the size of the cell, thethickness of the cell walls, the density of the sheet, etc. The size ofthe cell in the present invention is substantially not larger than 1 mm,and preferably not larger than 500 μm. In this case, the term"substantially not larger than 1 mm" means that the size of the greaterpart of the cells is not larger than 1 mm and does not mean that cellslarger than 1 mm must not exist in the foamed sheet. The same is truefor the thickness of the cell walls and the thickness of the cell wallsis substantially not thicker than 100 μm, preferably not thicker than 50μm, and more preferably not thicker than 20 μm.

Also, the size of the cell is generally larger than the thickness of thecell. The density of the foamed sheet is in the range of not more than1.1 g/cc, preferably not more than 0.8 g/cc, and more preferably from0.15 g/cc to 0.5 g/cc.

The important feature of the leather-like sheet of the present inventionis that the cell walls extend with irregular bending. This can be moreclearly seen in the enlarged cross section. FIG. 1 shows an enlargedschematic cross section of the leather-like PAT foamed sheet of thepresent invention cutting in the thickness direction of the sheet. FIG.2 is a microscopic photograph (magnification: about 35 times) showing anenlarged cross section of one example of the leather-like PAT foamedsheet of the present invention in the thickness direction.

In FIG. 1, numerals 1, 2, 3,--each shows a cell and numerals 11, 12,13,--each shows a cell wall dividing each cell, respectively. Each ofthe cell walls 11, 12, 13,--bends with various amplitudes of severaltimes larger than the thickness of the wall. These bendings include onepeaked bending as the case of folding back a paper and a bendingcomposed of many creases as the case of randomly smashing a paper in ahand and spreading the paper. The cell walls are teared and cracked inplaces.

On the other hand, an enlarged schematic cross section of a conventionalPAT foamed sheet in the thickness direction is as shown in FIG. 3.

In FIG. 3, numerals 21, 22, 23,--each shows a cell and numerals 31, 32,33,--each shows a cell wall each dividing each cell. Also, a microscopicphotograph (magnification: about 50 times) showing an enlarged crosssection of one example of a conventional PAT foamed sheet in thethickness direction is shown in FIG. 4.

As seen from FIGS. 3 and 4, cell walls 31, 32, 33--are curved to someextent. The curve between two adjacent specific cells always forms aconvex form or a concave form to one cell side and scarcely forms anintermingled state of convex forms and concave forms.

Thus, in a conventional foamed sheet, the cell walls are not irregularlybent. Also, even in a conventional foamed sheet, some of the cell wallsmay be cracked but the number of the cracks is far less than those ofthe leather-like sheet of the present invention.

As described above, the leather-like sheet of the present invention isutterly different from a conventional foamed sheet in the point that inthe former sheet, the cell walls are irregularly bent.

Also, the leather-like sheet of the present invention is different froma conventional PAT foamed sheet in the point of flexibility.

For example, as to an ordinary PAT foamed sheet which is a polyethyleneterephthalate sheet obtained by foaming using butane as a foaming agentand has a basis weight of 400 g/m², a thickness of 1.50 mm, and adensity of 0.27 g/cc, the flexibility measured by the Gurley flexibilitytest (as defined in JIS L 1096-1990--hereinafter the same) is 48,326 mg.On the other hand, as to the leather-like sheet of the present inventionmade of an ordinary PAT foamed sheet and having a basis weight of 397g/m², a thickness of 1.29 mm, and a density of 0.31 g/cc, theflexibility is 16,751 mg. Furthermore, by applying the similar test tovarious PAT foamed sheets, it has been confirmed that the flexibility ofconventional PAT foamed sheets is higher than 40,000 mg but theflexibility of the leather-like foamed sheets of the present inventionis not higher than 20,000 mg.

Accordingly, the leather-like PAT foamed sheet of the present inventionis specific in the point that the Gurley flexibility is not higher than20,000 mg.

Also, the PAT foamed sheet of the present invention shows a specificductility curve as compared to that of an ordinary PAT foamed sheet. Forexample, the above-described ordinary PAT foamed sheet, i.e., thepolyethylene terephthalate foamed sheet having a basis weight of 400g/m², a thickness of 1.50 mm and a density of 0.27 g/cc shows thetension-ductility relation shown by broken line A in FIG. 6, while theleather-like PAT foamed sheet of the present invention made from theaforesaid sheet having a basis weight of 397 g/m², a thickness of 1.29mm, and a density of 0.31 g/cc shows the tension-ductility line relationshown by one-dot and dash line B in FIG. 6.

That is, during the application of a tension of less than about 20kg/cm², the leather-like foamed sheet of the present invention increasesthe ductility in proportion to the tension and already shows theductility of about 50% at a tension of about 32 kg/cm². On the otherhand, the above-described ordinary PAT foamed sheet shows a ductility ofless than 5% and scarcely increases the ductility by only theapplication of the force up to about 50 kg/cm² and when a force of about60 kg/cm² is applied, the sheet first shows suddenly a large ductilityof 100% or higher.

As described above, the leather-like PAT foamed sheet of the presentinvention is specific in the point that the sheet already shows a largeductility of several % to several tens % by a small tension of up toabout 30 kg/cm².

Then, the production process for the leather-like PAT foamed sheetaccording to the present invention is explained. That is, according tothe production process, the uniformly and finely foamed sheet of PAT, inwhich the foamed sheet has cells having a diameter of substantially notlonger than 1 mm and the cell walls have a thickness of substantiallynot thicker than 100 μm, is used as the starting material and a force ofcompression or bending is applied to the foamed sheet at a temperatureof not higher than the glass transition point of the resin to extend thecell walls with wavy bending.

The foamed sheet of PAT which is used as the starting material in theproduction process of the present invention may be produced by anyproduction process. For example, the uniformly and finely foamed PATsheet obtained by foaming a PAT sheet in a reduced pressure zoneaccording to the teaching of JP-A-55-2045 described above can be used asthe starting material in the process of the present invention. However,a process of carrying out the foaming twice as will be described belowis preferred.

That is, in the process of carrying out the foaming twice,high-temperature PAT containing a foaming agent is extruded in a sheetform followed by quickly cooling directly thereafter to form a primaryfoamed sheet, and thereafter the primary foamed sheet is further foamedby heating to a temperature of at least 60° C. to provide a secondaryfoamed sheet.

For extruding PAT, as hitherto practiced, PAT is melted in an extruder,a foaming agent is charged under pressure in the extruder from anintermediate portion thereof, followed by kneading, and the kneadedmixture may be extruded in a sheet form from a die equipped to theextruder. At extruding the kneaded mixture, the form of the crosssection of the extruded product may be an annular form or a straightline form. When the cross section is annular, the cylindrical productthus extruded is opened by cutting to form a flat sheet.

The sheet directly after extruding is quickly cooled or quenched sincethe crystallinity of the primary foamed sheet is lowered no higher than30%. The quenching temperature is not higher than the glass transitionpoint of PAT.

In the present invention, by utilizing the fact that the secondaryfoamed sheet thus obtained has been foamed in such a state that thesheet has uniform and fine cells having a diameter of substantially notlonger than 1 mm and the cell walls defining the cells have a thicknessof substantially not thicker than 100 μm, the secondary foamed sheet iscompressed or bent at a temperature of not higher than the glasstransition point of PAT, thereby the cell walls become irregularly bent.

For compressing or bending the uniformly and finely foamed PAT sheet,various methods can be used.

For example, in the case of compressing the foamed sheet, thecompressing direction may be the thickness direction, the lengthwisedirection, or the width direction of the foamed sheet. However, thethickness direction can be easily compressed. In the case of compressingthe sheet to the thickness direction, the sheet may be compressedpartially or wholly but partial compressing is preferable.

Also, for bending the PAT foamed sheet, the bending direction may be anydirection but the lengthwise direction and the width direction of thesheet are preferred and the lengthwise direction is more preferred. Alsocompressing and bending may be applied singly but it is desirable toapply them as a proper combination thereof.

In practice, it is preferable to continuously carry out compressingand/or bending as a combination thereof. For example, there are a methodof partially compressing the surface of the foamed sheet by passing thesheet through embossing rolls having uneven surfaces, a method ofpartially compressing the foamed sheet to the thickness direction and atthe same time extending or loosening the sheet by passing the sheetthrough many pair rolls, and a method of bending the sheet to thelengthwise direction of the sheet and at the same time extending orloosening sheet to the lengthwise direction by passing zigzag the sheetthrough many rolls.

It is necessary that above-described compressing or bending is carriedout in the state that the temperature of the PAT foamed sheet is nothigher than the glass transition point of PAT. The glass transitionpoint of PAT differs according to the kinds of a dicarboxylic acid and adihydric alcohol constituting PAT but is generally in the range of from30° C. to 90° C. Accordingly, the PAT foamed sheet is compressed or bentat a temperature of 60° C. or lower.

Since PAT is a resin which is easily hydrolyzed at a high temperature,in the case of foaming PAT by extruding from an extruder, it isdesirable to previously dry PAT. For drying, it is preferred to use, forexample, a dehumidifying dryer. The drying condition in this case may bethat air having a dew point of, for example, -30° C. is heated to 160°C. and PAT is exposed to the air for about 4 hours.

As the foaming agent, various compounds can be used in the presentinvention. They can be generally classified into a solid compound whichis decomposed at a temperature of the softening point of PAT or higherto generate gases, a liquid which is vaporized in PAT by heating, and aninert gas which can be dissolved in PAT under pressure. The materialsclassified as above can be all used in the present invention.

Examples of the aforesaid solid compound are azodicarbonamide,dinitrosopentamethylenetetramine, hydrazocarbonamide, and sodiumbicarbonate. Examples of the aforesaid liquid which is vaporized in PATare saturated aliphatic hydrocarbons such as hexane, pentane, butane;saturated alicyclic hydrocarbons such as cyclohexane; aromatichydrocarbons such as benzene, xylene; and halogenated hydrocarbons suchas methylene chloride, Freon (a trade name, made by E. I. Du Pont deNemours and Company). Also, examples of the inert gas are carbon dioxideand nitrogen. Also, as other foaming agent, the high-molecular weightchain aromatic polycarbonate described in JP-A-59-135237 can be alsoused in the present invention.

When softened PAT containing a foaming agent is extruded from a die ofthe extruder, PAT is foamed. The foaming is the primary foaming in thepresent invention and the foamed product obtained is a primary foamedsheet. The primary foamed sheet has a low expansion ratio and usuallyhas a high density. The expansion ratio differs according to the form ofthe foamed product but is about 5 times only.

In the present invention, the primary foamed sheet thus obtained isquenched directly after forming and while the sheet is at a hightemperature to a temperature of the glass transition point of PAT orlower.

When the primary foamed sheet is quenched, the foamed sheet issolidified without a time of being crystallized and, hence, thecrystallinity thereof is low. The crystallinity differs according to theextent of quenching. For example, the crystallinity differs according tothe kind of a cooling medium, the temperature of a cooling medium, thecontact state with a cooling medium, etc.

When the primary foamed sheet prepared by extrusion is cooled by directcontact with water of normal temperature, the crystallinity of theprimary foamed sheet becomes several percent, and is usually lower than30%. However, when the primary foamed sheet prepared by extrusion isintroduced to a mold to adjust the form and the mold is not forciblycooled, the foamed sheet is not quenched and hence the crystallinitybecomes higher than 30%. In particular, the wall-thick primary foamedsheet has a crystallinity of higher than 30%. Accordingly, when theprimary foamed sheet is produced by an extruder, the primary foamedsheet is quenched by moving along a mold which is forcibly cooled.

For effectively quenching the primary foamed sheet, it is desirable thatthe thickness of the primary foamed sheet is as thin as possible suchthat the primary foamed sheet has a large surface area as compared tothe volume thereof. That is, it is desirable that the width of theprimary foamed sheet is increased as large as possible and the thicknessthereof is thinned to not more than 10 mm, in particular not more than 3mm. In this case, when the foamed sheet is extruded in the cylindricalform, a mandrel is disposed in a cylinder, the cylindrical foamed sheetis moved along the mandrel, and the mandrel is cooled with water as wellas the length of the mandrel is increased as long as possible.

On the other hand, when the foamed sheet is extruded in a tubular form,the sheet is advanced between a pair of rolls with cooling, and therolls are cooled with water as well as the rolls having a diameter of aslarge as possible are employed. Thus, the crystallinity of the primaryfoamed sheet is kept at no higher than 30%.

In general, the crystallinity of a resin can be measured by the density,the X-ray diffraction pattern, the nuclear magnetic resonance absorptionspectra, etc., but since a foamed resin sheet contains bubbles therein,the crystallinity thereof cannot be measured by these methods.Accordingly, the crystallinity of the primary foamed sheet of PAT mustbe measured by a thermal method such as a method of measuring the heatquantity of fusion and heat quantity of cold crystallization. Theprinciple utilizes the characteristic that when the PAT foamed sheet isheated at a constant rate, the crystals increase first and thereafterare melted but at the increase of the crystals, a heat is generated andat melting the heat of fusion is absorbed. Practically, the heatquantity of cold crystallization generated by crystallization and theheat quantity of fusion generated by melting are measured and thecrystallinity is calculated by comparing the measured value with theheat quantity of fusion shown by a standard sample of complete crystal.

For practically measuring the heat quantity of cold crystallization andheat quantity of fusion of the PAT foamed material, it is preferred toundergo the measurement by a method using a differential scanningcalorimeter. In the method using a differential scanning calorimeter, aheater for a sample to be measured and a heater for a standard samplework independently from each other and if a temperature differenceoccurs between them during constant-rate heating, one of the increasingor controlling mechanisms of heat flow acts automatically to nullify thetemperature difference and, hence, a difference of the heat flow ratesis directly recorded. The crystallinity is theoretically calculated bythe following equation:

Crystallinity(%)=(A-B)/C×100

A: Heat quantity of fusion per mol

B: Heat quantity of cold crystallization per mol

C: Heat quantity of fusion per mol of complete PAT crystal

In this case, the heat quantity of fusion per mol of a complete PATcrystal is described to be 26.9 KJ according to Kobunshi (High Molecule)Data Handbook, published by Baifukan and this value is used in thepresent invention.

In the present invention, the PAT primary foamed sheet having acrystallinity of not higher than 30% is subjected to the second time offoaming and this is secondary foaming. At the secondary foaming, theprimary foamed sheet is heated to a temperature of at least 60° C. Thereis no particular restriction on the heating means. That is, heating byconduction, heating by radiation, heating by convection, heating by highfrequency electric power, etc., can be employed in the presentinvention. Also, any heating media which do not corrode PAT can be used.In these heating methods, a method of heating the primary foamed sheetof PAT by bringing the foamed sheet into contact with a heated metal orheated air, or bringing the foamed sheet into contact with steam orheated water is preferred.

The heating time at secondary foaming is properly determined accordingto the nature and form of PAT and the kind and temperature of theheating medium. In general, when the temperature of the heating mediumis low, the heating time is prolonged and in the contrary, when thetemperature of the heating medium is high, the heating time isshortened. Also, when the thickness of the foamed sheet is thick, theheating time is prolonged and on the contrary, when the thickness isthin, the heating time is shortened.

When the primary foamed material is heated by contacting with a metalplate, the metal plate is heated to a temperature of from 60° C. to 200°C., and it is preferred to contact them for a time of at least 5seconds.

Also, when the primary foamed material is heated by bringing intocontact with air, it is desirable to place the primary foamed materialin an oven kept at an inside temperature of from 100° C. to 230° C. andto heat it for from 10 seconds to 5 minutes. In the case of heating theprimary foamed material with a metal plate or air, it is desirable toavoid the application of secondary foaming directly after primaryfoaming and to perform secondary foaming after allowing to stand thefoamed material after primary foaming for at least 24 hours and usuallyfor 3 days.

On the other hand, when the primary foamed material is heated bybringing into contact with steam, the primary foamed material can besubjected to secondary foaming directly after primary foaming. In thiscase, the temperature of steam or water is from 60° C. to 125° C. andthe contact time is from 10 seconds to 10 minutes. If the temperature ofsteam or water is over 125° C., there is a possibility of decomposingthe primary foamed material and, hence, it is recommended to avoid theemployment of such a high temperature.

For contacting the PAT foamed material with steam or heated water,various modes can be employed.

For example, as shown in FIG. 7, a primary foamed PAT material 41 may beimmersed in heated water 42. In FIG. 7, numeral 43 means a burner. Also,as shown in FIG. 8, a metallic wire net 44 is placed above the surfaceof heated water 42, a primary foamed material 41 is placed on the wirenet 44, and the foamed material 41 may be brought into contact withsteam 45 coming from water 42 by vaporization. Furthermore, as shown inFIG. 9, compressed steam 46 may be blown into a container 47 containinga foamed material 41.

Also, at the case of heating the foamed material by contact with heatedwater or steam, it is desirable the foamed material is placed in a moldto mold the material into a desired form. In the case of using a mold,heated water or steam is pressed in the mold containing the foamedmaterial to directly contact with the material.

As described above, when the PAT foamed material is heated by bringinginto contact with water heated above 60° C. or steam, the PAT foamedmaterial is secondarily foamed to become a foamed material having a lowdensity.

In general, heating by hot water or steam can be more easily applied tosecondary foaming than heating by air heating. By heating with hot wateror steam, the expansion ratio of the secondary foamed material is atleast 1.3 times and even more than 4 times in the case of a largeexpansion ratio. Furthermore, in the aforesaid case, foaming can beuniformly carried out and the secondary foamed material obtained hasfine and uniform cells. Thus, a foamed material having a good qualityand a low density can be obtained.

At the practice of the present invention, PAT may contain variousadditives. For example, PAT may contain a small amount of talc as a cellcontrolling agent, an antistatic agent, a pigment, a flame retarder, andfrom 0.5 to 5% by weight an acid dianhydride such as pyrromelliticanhydride, an epoxy compound such as diglycidyl terephthalate, an oxideof a metal belonging to group Ia or IIa of the periodic table, or sodiumcarbonate for improving the melting characteristics of PAT singly or asa mixture thereof.

Also, at the practice of the present invention, the secondary foamedmaterial obtained is kept at a high temperature of, e.g., at least 200°C. to increase the crystallinity of PAT, whereby the secondary foamedmaterial having a more improved heat resistance can be obtained.

The secondary foamed sheet thus obtained is compressed or bent asdescribed hereinbefore to provide a leather-like PAT foamed sheet.

Furthermore, in the present invention, the foamed sheet or leather-likesheet of PAT is immersed in an alkali solution for imparting a waterabsorption property or breathability thereto.

As the alkali solution, there are aqueous solutions of hydroxides,carbonates, and phosphates such as sodium hydroxide, potassiumhydroxide, calcium hydroxide, magnesium hydroxide, calcium carbonate,sodium carbonate, sodium phosphate, potassium phosphate, etc.

The pH of the aforesaid alkali solution is preferably at least 12.5 andwhen the foamed sheet or the leather-like sheet is immersed in such analkali solution and heated to a temperature of from 80° C. to 100° C.for a time of from 20 minutes to 30 minutes, the proportion ofcontinuous cells becomes from 45% to 80%. In the case of a foamed sheet,the sheet is further compressed or bent to provide a leather-like PATfoamed sheet.

Furthermore, by rubbing the surface of the sheet obtained by theaforesaid process with a sand paper, a brush, a file, etc., a backskin-form leather-like sheet can be obtained.

Also, a metal vapor deposition or coating can be applied to the surfaceof the PAT foamed sheet. Furthermore, before or after compressing orbending the foamed sheet, a colored film or a metal vapor-depositedfilm, etc., may be laminated on the foamed sheet.

As described above, according to the present invention, the leather-likePAT foamed sheet obtained has uniform and fine cells having a diameterof substantially not longer than 1 mm and the cell walls thereof have athickness of substantially not thicker than 100 μm. Thus, the foamedsheet has an appearance like a natural leather. Furthermore, since theleather-like foamed sheet of the present invention is a foamed sheetusing PAT as the material, the cell walls constructing the cells extendwith irregular bending to give flexibility as a sheet of not more than20,000 mg in the Gurley flexibility test, the foamed sheet has astrength and feeling like a natural leather. Moreover, since theleather-like foamed sheet of the present invention is made of PAT only,the foamed sheet has a characteristic that the production is easy.Accordingly, the leather-like sheet of the present invention can beexpected to be widely used in place of natural leathers. The presentinvention is very advantageous in this point.

Also, according to the present invention, by applying a force ofcompression or bending to a uniformly and finely foamed sheet of PAThaving cells of a diameter of substantially not longer than 1 mm andhaving cell walls having a thickness of substantially not thicker than100 μm at a temperature of not higher than the glass transition point ofPAT, the cell walls can be easily formed into an irregularly bent state,whereby a leather-like PAT foamed sheet can be easily made. Accordingly,by the process of the present invention, a leather-like sheet can becontinuously and easily supplied at a low cost. In this point, theproduction process of the present invention is also industrially veryadvantageous.

Furthermore, according to the production process of the presentinvention, since PAT containing a foaming agent is foamed by extrusionto form a foamed sheet and the foamed sheet at a high temperaturedirectly after extrusion is quenched below the glass transition point ofPAT to provide a primary foamed material having a crystallinity of PATof not higher than 30%, the crystallinity of the primary foamed sheet islow and, hence, the primary foamed sheet can be easily foamed by heatingagain. In the present invention, since such a primary foamed sheet isfoamed by heating to a temperature of at least 60° C., the secondaryfoamed sheet is formed at a high expansion ratio. Accordingly, thesecondary foamed sheet has uniform and fine cells having a diameter ofsubstantially not longer than 1 mm and has the cell walls having athickness of substantially not thicker than 100 μm. In the presentinvention, since a force of compression or bending is applied to such asecondary foamed sheet at a temperature of not higher than the glasstransition point of PAT, a leather-like sheet can be easily produced asdescribed above. Accordingly, by the production process of the presentinvention, a leather-like sheet can be continuously and more easilysupplied at a low cost. The production process of the present inventionis also very advantageous in this point.

Also, according to the present invention, by immersing the leather-likefoamed sheet or intermediate foamed sheet in an alkali solution, thecells thereof become continuous cells, whereby breathability and waterabsorbing property can be easily imparted to the leather-like sheet. Asdescribed above, the foamed sheet having the gas permeability and waterabsorbing property is more similar to a natural leather in terms of theproperties.

Also, the leather-like PAT foamed sheet of the present invention doesnot cause cracking from the seams by needles and has a sufficient sewingstrength. Thus, by sewing the leather-like foamed sheet of the presentinvention, bags, suitcases, clothes, etc., can be easily made.

Then, by referring to the examples of the present invention andcomparison examples, the merits of the present invention are practicallyexplained, in which parts and percents are all by weight unlessotherwise indicated.

EXAMPLE 1

(Production of primary foamed sheet)

By using polyethylene terephthalate ("TR 8580", a trade name made byTeijin Limited) as PAT, the following composition was prepared.

    ______________________________________                                        PAT                    100    parts                                           Talc                   0.6    part                                            Pyromellitic Anhydride 0.3    part                                            Sodium Carbonate       0.1    part                                            ______________________________________                                    

The aforesaid composition was placed in an extruder having a diameter of65 mm, and the temperature of the supply portion of the extruder was setat 280° C., the temperature of the compressive portion at 285° C., thetemperature of the melting portion and the temperature of the headportion at 275° C., the temperature of the mold at 270° C., and thescrew revolution number at 25 r.p.m., respectively. Also, while pressingin 1.3 parts of butane into the composition as a foaming agent from theintermediate portion of the extruder, the composition was extruded. Anannular die having an inside diameter of 60 mm and a slit width of 0.4mm was used. Also, a plug having a diameter of 205 mm and a length of740 mm was equipped to the top of the die and the plug was quenched withwater of 25° C.

The cylindrical foamed sheet thus obtained was opened by cutting toprovide a flat sheet which was then once wound into a roll form toprovide a primary foamed sheet. The primary foamed sheet had a basisweight of 400 g/m², a thickness of 1.50 mm, a density of 0.27 g/cc, amean cell size of 320 μm, and a mean cell wall thickness of 30 μm. Also,the crystallinity of the primary foamed sheet was 8% and the flexibilitythereof was 48,326 mg.

(Production of secondary foamed sheet)

Thereafter, the primary foamed sheet obtained was further foamed bybringing the sheet into contact with steam of 100° C. for 5 minutes andthe foamed sheet was allowed to cool at room temperature to provide asecondary foamed sheet. The secondary foamed sheet thus obtained had abasis weight of 400 g/m², a thickness of 5.6 mm, a density of 0.07 g/cc,a mean cell size of 560 μm, and a mean cell wall thickness of about 20μm. Also, the flexibility of the secondary foamed sheet was 58,488 mg.

(Treatment of secondary foamed sheet)

For softening the secondary foamed sheet obtained above, the foamedsheet was compressed by embossing rolls at room temperature (25° C.). Asthe embossing rolls, a pair of rolls each having a diameter of 14 cmwere used and the gap between the rolls was 0.3 mm. On the surface ofthe roll, grooves having a width of 0.2 mm and a depth of 0.5 mm wereformed at an interval of 1 mm in both the axial direction and theperipheral direction of the roll. Also, the revolution rate of the rollswas 3 r.p.m. and the secondary foamed sheet was passed once through therolls.

The secondary foamed sheet thus compressed was a good leather-likesheet. The leather-like sheet had a basis weight of 404 g/m², athickness of 1.56 mm, a density of 0.25 g/cc, a maximum point tensilestrength of 4.90 kg/cm, a tensile elongation at break of 39.6%, amaximum tear strength of 1.12 kg, and a flexibility of 12,118 mg. Also,when the sheet was tried to be teared by hand, the sheet could not beteared.

EXAMPLE 2

In this example, the primary foamed sheet obtained in Example 1 wasimmersed in boiling water to cause secondary foaming and the secondaryfoamed sheet was compressed by rolls each having a smooth surface toprovide a leather-like sheet. The details were as follows.

(Production of secondary foamed sheet)

The primary foamed sheet was immersed in boiling water for 2 minutes tocause secondary foaming and then allowed to cool at room temperature toprovide a secondary foamed sheet. The secondary foamed sheet had a basisweight of 400 g/m², a thickness of 5.2 mm, a density of 0.07 g/cc, amean cell size of 560 μm, and a mean cell wall thickness of about 20 μm.

(Treatment of secondary foamed sheet)

For softening the secondary foamed sheet obtained above, the sheet wascompressed by rolls. The rolls each having a diameter of 14 cm and asmooth surface were used and the rolls were faced with each other at agap Of 0.3 mm. The rolls were revolved at a rate of 3 r.p.m. and thesecondary formed sheet was passed once through the rolls. On the surfaceof the foamed sheet were formed small creasy unevennesses to show aleather-like appearance. The leather-like sheet obtained had a basisweight of 404 g/m², a thickness of 1.43 mm, a density of 0.28 g/cc, anda flexibility of 12,296 mg.

EXAMPLE 3

In this example, the secondary foamed sheet obtained in Example 1 wascompressed by a press to provide a leather-like sheet. The details wereas follows.

For softening the secondary foamed sheet, the sheet was inserted betweenaluminum plates at room temperature (25° C.) and compressed by press.The aluminum plates had an area of 30 cm×30 cm and had grooves having adepth of 0.5 mm formed irregularly on the smooth surface in variousdirections. The sheet was pressed by applying thereto a pressure of 250kg/cm² for 10 seconds.

Thus, a leather-like foamed sheet was obtained and the sheet had a basisweight of 404 g/m², a thickness of 2.07 mm, a density of 0.20 g/cc, anda flexibility of 11,672 mg.

EXAMPLE 4

In the example, a foamed sheet was repeatedly compressed. The detailswere as follows.

The secondary foamed sheet obtained in Example 1 was passed twicethrough the embossing rolls used in Example 1. In this case, therevolution rate of the rolls was 3 r.p.m. as in Example 1. Theleather-like sheet became more rich in flexibility. The leather-likesheet obtained had a basis weight of 404 g/m², a thickness of 1.41 mm, adensity of 0.28 g/cc, and a flexibility of 10,893 mg.

EXAMPLE 5

In this example, by rubbing the surface of the leather-like sheetobtained in Example 1 with a sand paper, a back skin-like sheet wasobtained. The details were as follows.

A sand paper of #400 was used. The sand paper was placed on the surfaceof the leather-like sheet obtained in Example 1 and while pressing thesand paper, the surface of the sheet was rubbed 6 times with the sandpaper, whereby the surface of the leather-like sheet became backskin-like. The back skin-like sheet obtained had a basis weight of 404g/m², a thickness of 1.56 mm, a density of 0.25 g/cc, and a flexibilityof 11,472 mg.

EXAMPLE 6

In this example, the secondary foamed sheet obtained in Example 1 waspassed through the embossing rolls and thereafter, the secondary foamedsheet was crumpled. The details were as follows.

The secondary foamed sheet obtained in Example 1 was passed once throughthe embossing rolls used in Example 1. The leather-like sheet of 30cm×30 cm thus obtained was two-folded, placed between two aluminumplates, and both the ends of the sheet were fixed to the surface of eachaluminum plate with an adhesive. The aluminum plates were placedhorizontally, a weight of 5 kg was placed thereon, the upper aluminumplate was reciprocated at 6 times/minute, and the sheet was crumpled byreciprocating the aluminum plate 30 times.

The sheet obtained was a leather-like sheet enriched with flexibilityand the sheet had a basis weight of 404 g/m², a thickness of 1.05 mm, adensity of 0.24 g/cc, and a flexibility of 8,862 mg.

EXAMPLE 7

(Production of primary foamed sheet)

By using polyethylene terephthalate ("TR 8580", trade name, made byTeijin Limited) as PAT, the following composition was prepared.

    ______________________________________                                        PAT                    100    parts                                           Talc                   0.6    part                                            Pyromellitic Anhydride 0.30   part                                            Sodium Carbonate       0.1    part                                            ______________________________________                                    

The aforesaid composition was placed in an extruder having a diameter of65 mm and the temperature of the supply portion of the extruder was setat 280° C., the temperature of the compressing portion at 285° C., thetemperature of the melting portion at 270° C., the temperature of thehead portion at 275° C., the temperature of the die at 270° C., and therevolution number of the screw at 25 r.p.m., respectively. Also, whilepressing 1.1 parts of butane as a foaming agent into the compositionfrom the intermediate portion of the extruder, the composition wasextruded. In this case, an annular die having an inside diameter of 60mm and a slit width of 0.4 mm was used. Also, a plug having a diameterof 205 mm and a length of 740 mm was equipped to the top of the die andthe plug was quenched with water of 25° C.

The cylindrical foamed sheet thus obtained was opened by cutting toprovide a flat sheet which was then once wound into a roll form toprovide a primary foamed sheet. The primary foamed sheet had a thicknessof 1.1 mm and a density of 0.30 g/cc.

(Treatment with alkali solution)

The foamed sheet obtained above was immersed in a 30% sodium hydroxideaqueous solution for 30 minutes at 80° C. The continuous cell ratio ofthe foamed sheet obtained was 48% and the water absorption was 54%. Thesodium hydroxide aqueous solution immersed in the foamed sheet was thenremoved by a large quantity of water.

On the other hand, the continuous cell ratio of a foamed sheet which hadnot been treated with the alkali solution was 4% and the waterabsorption was 0.7%.

(Production of secondary foamed sheet)

The primary foamed sheet thus treated with an alkali solution wasbrought into contact with steam of 100° C. for 3 minutes to further foamit. There was thus obtained a secondary foamed sheet.

The secondary foamed sheet had a thickness of 3.8 mm, a density of 0.087g/cc, and a crystallinity of 22%. A microscopic photograph(magnification: about 35 times) to show a cross section of the cellstructure of this foamed sheet in the thickness direction is given inFIG. 5(b).

(Treatment of secondary foamed sheet)

For softening the secondary foamed sheet obtained above, the sheet waspassed twice through the embossing rolls used in Example 1. In thiscase, the revolution rate of the rolls was 3 r.p.m. as in Example 1. Theleather-like sheet obtained was rich in flexibility and had a basisweight of 326 g/m², a thickness of 1.3 mm, a density of 0.25 g/cc, and aflexibility of 7,976 mg.

EXAMPLE 8

(Production of primary foamed sheet)

A primary foamed sheet was prepared in the same manner as in Example 7.

(Production of secondary foamed sheet)

The primary foamed sheet was brought into contact with steam of 100° C.for 5 minutes to cause secondary foaming and then allowed to cool atroom temperature to provide a secondary foamed sheet. The secondaryfoamed sheet had a basis weight of 330 g/m², a thickness of 4.1 mm, anda density of 0.08 g/cc (The time difference between the production ofthe primary foamed sheet and the production of the secondary foamedsheet was 2 weeks.)

(Treatment of secondary foamed sheet)

For softening the secondary foamed sheet obtained above, the foamedsheet was compressed by embossing tools. As the embossing rolls, a pairof rolls each having a diameter of 14 cm were used and the gap betweenthe rolls was 0.3 mm. On the surface of the roll, grooves having a widthof 0.2 mm and a depth of 0.5 mm were formed at an interval of 1 mm inboth the axial direction and the peripheral direction of the roll. Also,the revolution rate of the rolls was 3 r.p.m. and the secondary foamedsheet was passed once through the rolls.

(Treatment with alkali solution)

The thus obtained leather-like sheet was treated with an alkali solutionin the same manner as in Example 7 and further compressed by embossingrolls in the same manner as described above.

The leather-like sheet obtained had a density of 0.25 g/cc, a continuouscell ratio of 63%, a water absorption of 92%, and a flexibility of 7,608mg.

In addition, in the above-described examples, the water absorption wascalculated from ratio of the original weight of a sample and theincreased weight after absorbing water according to Method A as definedin JIS K7209, and the continuous cell ratio was calculated according toASTM D2856.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. A process of producing a leather-likethermoplastic polyester series resin sheet having a density of 0.15-0.5g/cm³ and a flexibility of not more than 20,000 mg in the Gurleyflexibility test, which comprises foaming a thermoplastic polyesterseries resin containing a foaming agent by extrusion to form a foamedsheet, quenching the foamed sheet at a high temperature directly afterextrusion to a temperature below the glass transition point of the resinto provide a primarily foamed sheet having a crystallinity of the resinof not higher than 30%, heating the primarily foamed sheet to atemperature of at least 60° C. to provide a secondarily foamed sheethaving cells of a diameter of substantially not greater than 1 mm andhaving a cell wall thickness of substantially not greater than 100 μm,and thereafter irregularly bending said cell walls by applying a forceof compression or bending to the secondarily foamed sheet at atemperature below the glass transition point of the resin.
 2. Theprocess of claim 1, wherein the force of compression or bending isapplied to the secondarily foamed sheet by passing the secondarilyfoamed sheet through at least a pair of rolls.
 3. The process of claim2, wherein said at least a pair of rolls are embossing rolls.
 4. Theprocess of claim 1, wherein the secondarily foamed sheet is compressedby a press.
 5. A process of producing a leather-like thermoplasticpolyester series resin sheet having a density of 0.15-0.5 g/cm³ and aflexibility of not more than 20,000 mg in the Gurley flexibility test,which consists essentially of applying a force of compression or bendingto a uniformly and finely foamed sheet comprising a thermoplasticpolyester series resin having cells of a diameter of substantially notlonger than 1 mm and a cell wall thickness of not thicker than 100 μm toirregularly bend said cell walls and then immersing the foamed sheet ina strong alkali solution to form continuous cells, the proportion ofcontinuous cells in said foamed sheet being 30-80%.
 6. The process ofclaim 5, wherein the pH of the strong alkali solution is at least 12.5.7. A process of producing a leather-like thermoplastic polyester seriesresin sheet having a density of 0.15-0.5 g/cm³ and a flexibility notmore than 20,000 mg in the Gurley flexibility test, which consistsessentially of immersing a uniformly and finely foamed sheet composed ofa thermoplastic polyester series resin having cells of a diameter ofsubstantially not longer than 1 mm and a cell wall thickness ofsubstantially not thicker than 100 μm in a strong alkali solution toform continuous cells and then irregularly bending said cell walls byapplying a force of compression or bending to the aforesaid foamedsheet.